My (M23) relationship with my gf (F24) began almost a year ago whilst we were still both at university. She always got on well with guys and was very flirty but one time I convinced to go on a date and things just kept on going well for us from there becoming official about a month later. Cut forward to 4 months ago and things had begun to deteriorate a little with her becoming more needy and borderline controlling of how much I went out and who I saw whilst she began to go out and party more without me at the same time. I began to hear things from other friends about how they'd seen her flirting with other guys but I simply didn't want to believe it. Then about a month and a half ago she was meant to be coming back to mine after one of her solo nights out but she didn't arrive until the next morning. She said she just ended up staying with a friend but I had a gut feeling something was off. She then went for a shower and I don't know why but I started to look through her bag and ended up finding her thong from the night before...and it was soaked in guys cum. To this day I don't know why I did what I did next... I put them to my mouth and begun to taste it, it didn't taste good but for some reason that was my first instinct instead of going straight to her and asking what the hell had happened. Perhaps if I did I would not be in the situation I am in now as the next second she finished in the shower and came out into my room, catching me tasting another guys cum off of her thong, she imitates began to take photos and shouted something along the lines of "what the fuck are you doing with my underwear you creep". I was so taken aback about being caught doing that I didn't even think to question her on it. She then began to say how she couldn't wait to show all her friends what a creep she'd found me out to be, I couldn't believe it! I began begging her to delete the photos as they would ruin my reputation. I never knee she was this cruel until that moment and this was the point at which our relationship switched. I later found out from her that she had been cheating on me for a while before this but this was the one time she'd slipped up and I'd obviously found out, she didn't want to break up with me but knew she had to double down or risk me breaking up with her. So in exchange for her not showing anyone the photos she told me to strip naked and put on the cum soaked thong... reluctantly I did. Then she began to take more photos, the start of what is her now very substantial blackmail folder of content she has of me. She then system talking, telling me how I'd been inadequate in bed and if I'd have been better she'd never have had to cheat, but in the next breath she was saying how much she loved me and never wanted me to leave. It wasn't until a week later when I saw her again that she came with a proposal, and if I agreed she would keep all of the blackmail material to herself:

1. We would stay together and not break up
2. Twice a month she could see another guy because as she said I was 'inadequate'
3. In an attempt to help me have some more passion in the bedroom I would wear a chastity cage when not at uni/work or playing sport (how she found out about cages is a story for another time... but it involves another guy)
4. If she so chooses and I agree she can make changes to these original rules

At the end of the day I had no choice but to agree. I still loved her and I couldn't bear the thought of thoes pictures getting out. And since then I have been her chastity cuck bf.

Plenty more developments have happened in the month and a half since then and if people are interested I'll happily write another post.